He signed a three-year contract extension with San Francisco on Thursday, ensuring the team can keep a key member of its running game through the 2017 season.

The do-everything Miller was lost for the season Dec. 15 with a left shoulder blade injury sustained during a win at Tampa Bay — and his absence left a huge void. San Francisco greatly missed his contributions the rest of the way before losing to the eventual Super Bowl champion and rival Seattle Seahawks in the NFC title game.

Miller had a tough time using the arm after the game. He took a hard hit from Buccaneers safety Keith Tandy after a 10-yard catch midway through the fourth quarter.

He has been training and rehabbing at 49ers headquarters and is ready for a full offseason program with his teammates.

"It's going great. I had a complete, full, healthy recovery," Miller said. "It's looking good. I'll be completely ready and 100 percent come April 21 and have a great offseason. I'm a full participant right now. If we were to start today I'd be ready to go."

The 26-year-old Miller has been a key blocker for 1,000-yard rusher Frank Gore, and Miller had 25 catches for 243 yards along with seven carries for 13 yards last season. He worked to convert from college defensive end to NFL fullback. Gore has long credited Miller with allowing him to break big gains.

"Bruce is a hard-working, dedicated young player that has done an outstanding job for us, both on and off the field," 49ers general manager Trent Baalke said. "He is one of those players that contributes a lot more to the team and to the community than he gets credit for, yet wouldn't have it any other way. We are pleased that Bruce has chosen to extend his stay with the 49ers."

During the 2011 season as a rookie, Miller caught the 49ers' lone touchdown in a 19-11 win at Washington and emerged as a feel-good story in coach Jim Harbaugh's first season.

Harbaugh has appreciated Miller from Day 1.

Especially considering Miller was open to the idea of converting from defensive end to fullback. The move gave him a regular job with the 49ers.

"It's definitely been a long journey and a lot of hard work, but it's been a blast. I couldn't have planned it out any better for me," Miller said. "Three years as a part of this organization, I couldn't ask for anything more. Excited for what's to come."

Miller was a dominant defensive end for Central Florida, earning Conference USA Defensive Player of the Year honors while becoming the school's all-time sacks leader with 36.

"I've had so much help, especially when I first came in, transitioning to offense," Miller said. "Him having the confidence of putting me out on the field and letting me play that first year was big."

While Miller has quietly emerged supporting Gore and the running backs in so many ways, he has become one of the players the Niners count on most.

San Francisco has increased his role and load each season, and he would love to take on even more.

"For me, it's wanting to definitely just be a part and do whatever I can to help our offense be successful. Every year it's been evolving and more on my plate," Miller said. "Hopefully, more additions and a little more playing time for me."